Process for producing rutile

ABSTRACT

Ilmenite ores are thermally oxidized to form a stable crystal state type intermediate product consisting of two phases (pseudobrookite+rutile) in a stable crystal phase, and the resultant intermediate product can be subjected to a magnetic separation procedure to selectively collect the rutile phase.

FIELD OF THE INVENTION

This invention relates to techniques for producing TiO₂-high qualitysynthetic rutile by upgrading a titanium- and iron-containing ilmeniteore.

BACKGROUND OF THE INVENTION

In spite of its being lightweight, titanium (Ti) is not only veryexcellent in strength, thermal stability and corrosion resistance butalso highly biocompatible. Ti has intrinsic characteristics such as anonmagnetic property, and is therefore utilized in the form of metalsand/or alloys in various fields covering, for example, from aircraftmaterials, anticorrosive chemical equipment materials and medicalinstruments to golf clubs and frames of spectacles, and further in theform of oxides as a pigment and lately as a photocatalyst and in otherenvironmental fields. Currently the starting materials used in theproduction of titanium are natural rutile, high-quality syntheticrutile, and high-titanium slag. Titanium is mainly obtained by the Krollprocess which comprises chlorinating titanium oxide contained thereinand reducing the resulting titanium chloride with metallic magnesium.Recently, a process comprising the direct reduction of titanium oxidehas also been proposed.

In any case, there is a problem of worldwide progressive exhaustion ofthe natural rutile resources with high TiO₂ quality (generally with aTiO₂ content of 93 to 96%) which are currently used as the materials forthe production of titanium. Therefore, it is desired that a new processshould be established for the production of synthetic rutile having ahigh TiO₂ content comparable to that of natural rutile by upgradingabundantly occurring natural ilmenite ores (mainly composed of iron andtitanium oxides, generally containing TiO₂ at 30 to 65% and containingother oxide impurities, including manganese, chromium, vanadium,magnesium, calcium, silicon, aluminum and other oxides).

Ilmenite ores are found naturally in the form of ilmenite (FeTiO₃),weathered ilmenite, and mixtures thereof.

The artificial rutile production techniques used in the prior artinclude the titanium slag process, selective acid leaching process andselective chlorination process. The titanium slag process comprisessubjecting the ilmenite ore to thermal reduction with carbon as areducing agent, separating the reaction mixture into a molten iron phaseand a molten titanate slag phase, and subjecting the titaniumoxide-enriched titanate slag to sulfuric acid leaching treatment and thelike to give artificial rutile. In this process, when the ilmenite oreis thermally reduced with a carbonaceous material, the iron oxidecomponent in the ore is first preferentially reduced, resulting in theformation of molten iron and molten titanate slag. The titanate slagthus derived from the ore and now enriched in titanium oxide issubjected to acid leaching treatment to give rutile.

In the selective acid leaching process, the ilmenite ore is heated withconcentrated sulfuric acid to give a product, to which is then addediron while adding water or diluted sulfuric acid, thereby causingreduction occurrence. In the next step, after the resulting FeSO₄ isremoved, the liquid residue is decomposed to recover titanium in theform of hydroxide, and the hydroxide is oxidized to give rutile.

The selective chlorination process comprises the steps of heating theilmenite ore in an oxidizing atmosphere at 1173 to 1223K to oxidize FeOto Fe₂O₃, adding ground coke at about 8%, reacting the mixture withchlorine gas in a fluidized bed chlorination furnace at 1223K, removingthe resulting iron chloride, and recovering synthetic rutile from theresultant product by magnetic separation or flotation.

In utilizing the natural ilmenite ore, in which oxidation has usuallyprogressed as a result of weathering, it is essential to have sufficientdata concerning the Fe₂O₃—FeTiO₃—TiO₂ region corresponding to the highoxygen side of the Ti—Fe—O system, for example the equilibriumrelationships, phase diagrams and oxygen isobars concerning the Ti—Fe—Oternary system phases and the oxygen partial pressure. Under presentconditions, however, sufficient findings about said region have not beenobtained as yet. Meanwhile, Webster and Bright (Non-Patent Document 1)and Taylor (Non-Patent Document 2) have reported on the high oxygen sideof the FeO—Fe₂O₃—TiO₂ system at 1473K and 1573K, respectively. However,considering the relatively low temperature processing to be employedfrom the industrial applicability viewpoint, their data cannot beutilized.

[Non-Patent Document 1] A. H. Webster and N. F. H. Bright, J. Am. Ceram.Soc., 44(3), pp. 110-116 (1961)

[Non-Patent Document 2] R. W. Taylor, Am. Miner., 49(7-8), pp. 1016-1030(1964)

For developing an industrial upgrading process for the production ofsynthetic rutile having a high TiO₂ content comparable to that ofnatural rutile via removing iron from natural ilmenite ores abundantlyoccurring as titanium resources, it is necessary to analyze details ofthe phase equilibrium of the Fe₂O₃—FeTiO₃—TiO₂ system corresponding tothe high oxygen content region of the Ti—Fe—O system at relatively lowtemperatures and further details of the phase relations as well as theoxygen partial pressure-related equilibrium phase diagram in anindustrially applicable temperature range such as around 1373K.

In the conventional processes for producing synthetic rutile byupgrading ilmenite ores, the use of concentrated sulfuric acid andothers secondarily leads to the formation of large quantities of acidicliquid wastes. From the viewpoint of environmental issues, this isbecoming more and more unacceptable. Hence, there is a demand for anescape from those processes which produce, as byproducts, some or othersubstances possibly causing an environmental problem or causing adisposal problem and for the development of an advantageous process fromthe cost viewpoint. In particular, those conventional processes whichuse a reducing agent and the like or a strong acid have problems: forexample, the use of a relatively expensive reagent is required, therecovery of such expensive reagents is required, environmentalpollutants are produced in large quantities, byproduct treatment costsmoney, and/or installation of treatment equipment entails a hugeexpenditure.

Therefore, there is still a need to develop techniques for producinghigh-quality synthetic rutile products at low cost in a simplifiedprocess while decreasing the byproduct formation.

SUMMARY OF THE INVENTION

Naturally occurring ilmenite ores have a composition mostly comprisingtwo coexisting phases, i.e., ilmenite (FeTiO₃: α phase) andpseudobrookite (Fe₂TiO₅: P phase), or further rarely coexisting withrutile (TiO₂; R phase). When such ilmenite ores are reduced with acarbonaceous material under heating, their composition shifts to thethree phase-coexisting region (Fe+R+α) i.e., metallic iron, rutile andilmenite coexist with the progress of oxygen removal. Since iron andtitanium oxide can form molten slag within the range of hightemperatures at which iron is molten, most of titanium oxide in the oresis concentrated in that slag. This is the basic principle of theconventional titanium slag process.

The present inventors have made an extensive investigation on the phaseequilibrium in a relatively low temperature range. As a result, thepresent inventors have succeeded in finding that ilmenite ores can beoxidized at a relatively low temperature around 1100° C. (about 1373K)to form, in the ore, a stable crystal phase product composed of twophases pseudobrookite (P phase) and rutile (R phase) in a stable crystalphase, and the resultant product wherein said (P+R) phases are in thephase of stable crystals can be separated by application of a strongmagnetic field into the P phase and the R phase. Consequently, they haveachieved the present invention.

In an aspect, the present invention provides the following:

(1) A process for upgrading an ilmenite ore to produce high-qualitysynthetic rutile products which comprises the steps of:

(a) oxidizing the ilmenite ore to form a stable crystal state typeintermediate product consisting of two phases, (pseudobrookite+rutile),in a stable crystal phase,

(b) then subjecting the resultant intermediate product to a magneticseparation procedure to separate a rutile phase product as a fractionsubstantially nonresponsive to a magnetic field, and

(c) recovering the rutile phase product from others.

(2) The process according to the above (1), wherein the ilmenite oreoxidation is carried out without allowing any reductive reaction.

(3) The process according to the above (1) or (2), wherein the startingore almost consisting of monophase ilmenite grains is oxidized to form astable crystal state type intermediate product consisting of two phases,(pseudobrookite+rutile), in a stable crystal phase.

(4) The process according to any of the above (1) to (3), wherein thestarting ore is oxidized under non-reducing conditions to form a stablecrystal state type intermediate product consisting of two phases,(pseudobrookite+rutile), in a stable crystal phase, the resultantintermediate product is then pulverized so as to give (1) fine particlessubstantially composed of pseudobrookite and (2) fine particlessubstantially composed of rutile, the pulverized fine particles aresubjected to a magnetic separation procedure to separate a rutile phaseproduct as a fraction substantially nonresponsive to a magnetic field,and the high-quality rutile phase product was recovered.

(5) The process according to the above (4), wherein the pulverizationstep is carried out once or repeated plural times.

(6) The process according to any of the above (1) to (5), whereinproducts from the ilmenite ore oxidation step is subjected, underconditions free from any reduction step, to a magnetic separationprocedure wherein products composed of rutile are separated fromproducts composed of pseudobrookite.

(7) The process according to any of the above (1) to (6), wherein themagnetic separation procedure is carried out once or repeated pluraltimes.

(8) The process according to any of the above (1) to (7), wherein theoxidation is carried out at a temperature ranging from 700 to 1200° C.

(9) The process according to any of the above (1) to (8), wherein themagnetic separation procedure is carried out under magnetic conditionsranging from 0.4 to 10 teslas.

(10) The process according to any of the above (1) to (9), wherein thatthe resultant rutile phase product has a TiO₂ level of not lower than90%.

(11) The process according to any of the above (1) to (10), wherein theprocess until the production of high-quality synthetic rutile productsfrom the ilmenite ore is substantially free of any reduction.

The inventive techniques require a less amount of energy to be consumed,and no carbonaceous material, etc. as a reducing agent, as compared tothe conventional titanium slag process. For separating rutile from thereaction products, no special treatment (such as sulfuric acid leachingwhich is essential in the titanium slag process) is required in theinventive process.

The above objects and other objects, features, advantages, and aspectsof the present invention are readily apparent to those skilled in theart from the following disclosures. It should be understood, however,that the description of the specification including the following bestmodes of carrying out the invention, examples, etc. is illustratingpreferred embodiments of the present invention and given only forexplanation thereof. It will become apparent to the skilled in the artthat a great number of variations and/or alterations (or modifications)of this invention may be made based on knowledge from the disclosure inthe following parts and other parts of the specification withoutdeparting from the spirit and scope thereof as disclosed herein. All ofthe patent publications and reference documents cited herein forillustrative purposes are hereby incorporated by reference into thepresent disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the formulations of samples used in determinations of theFe₂O₃—FeTiO₃—TiO₂ system phase equilibrium at 1373K. Each circleaccompanied by a numeral indicates the point corresponding to one ofrespective test sample formulations, and each numeral indicates a samplenumber.

FIG. 2 shows the experimental results of Sample No. 7 for relationsbetween the heating time and the compositions of the a phase(hematite-ilmenite s.s.) and the P phase (pseudobrookite s.s.) at 1373K.

FIG. 3 shows a phase diagram of the Fe₂O₃—FeTiO₃—TiO₂ system at 1373K.

FIG. 4 shows relations between the Ti—Fe—O system compositions and theoxygen partial pressure at 1373K.

FIG. 5 shows representatives of natural ilmenite ore compositions in theFe₂O₃—FeTiO₃—TiO₂ system phase diagram.

FIG. 6 shows the experimental results for the magnetic susceptibilitiesof ilmenite ore-derived P and R phases.

FIG. 7 shows the X ray diffraction analysis results of the productobtained by thermal oxidation of an ilmenite ore, in comparison with theresults prior to heat treatment.

BEST MODES OF CARRYING OUT THE INVENTION

Upon careful study of the Fe₂O₃—FeTiO₃—TiO₂ system phase diagram at1373K (FIG. 3), revealed by the present inventors, it becomes apparentthat naturally occurring ilmenite ores are distributed along the ternaryα+R+P phase-coexisting region centering around the α phase, as shown inFIG. 5. It also becomes apparent that oxidation of the ore leads to anincrease in only the oxygen content of the ore composition, wherein thecomposition shifts obliquely from the lower right corner of the triangleshown in FIG. 5 toward the upper left at an angle of about 45 degrees.Thus, it is understood that oxidation of the ilmenite ore will drive thesame into the binary R+P phase-coexisting region. If this productcomposed of the two phases (R+P) is separated into the R phase and the Pphase, rutile is expected to be produced in a relatively easy mannerwithout consuming a large amount of energy in treating the ore,differently from the prior art. Further, paying attention to the R and Pphases, the present inventors have examined their magneticsusceptibilities. As a result, the present inventors have succeeded infinding that the magnetic susceptibility of the P phase is almostcomparable to that of ferric oxide (Fe₂O₃) while the magneticsusceptibility of the R phase is negligibly small, as shown in FIG. 6.Thus, the P phase can be magnetically attracted and removed from theoxidized ilmenite ore in a strong magnetic field of about several teslaswherein the R phase (rutile) can be selectively separated and recovered.

The raw materials to be used in the process for producing upgradedrutile according to the invention include those capable of givinghigh-quality synthetic rutile products each having a TiO₂ content of atleast 90% or higher (in some instances, rutile products each having aTiO₂ content exceeding at least 92%). They also include natural ilmeniteores containing iron and titanium, for example, those ores having ann_(Ti)/(n_(Fe)+n_(Ti)) ratio of 0.35 to 0.65, more preferably ann_(Ti)/(n_(Fe)+n_(Ti)) ratio of 0.5 to 0.65, wherein n_(i) stands for amolar number.

In the inventive upgrading process, the raw material ore is firstsubjected to oxidative treatment. The oxidative treatment is notparticularly restricted, but may be carried out under any condition aslong as it is capable of substantially forming at least the two phases(pseudobrookite+rutile) in a stable crystal phase. For the oxidation, avariety of appropriate conditions can be selected. Said oxidation can becarried out via bringing for example the raw material ore and anoxidizing agent into contact with each other at an elevated temperature,for example, at a temperature ranging from about 700 to 1200° C.,preferably from about 900 to 1100° C.

Preferable oxidation conditions are those capable of providing thesufficient growth and profitable formation of the rutile phase. However,since it is also required to save as much energy as possible for theheat treatment, it is desirable to select treatment conditions at as lowtemperature as possible in as short time as possible. Although a highertemperature and a longer retention time are, in general, favorable forthe growth of rutile phase crystal grains (rustle crystal particles), itis preferable to select suitably optimal conditions depending on miningplaces in which starting ores are produced, because titanium is one ofthe most abundant metal elements on the earth and ilmenite ores aremined in various places, accompanying a subtle distinction in coexistingimpurity elements according to the mining place. Preferred treatmentconditions can be determined after clarifying the relations between heattreatment temperature, retention time, etc., and microstructures. Theoxidation includes preferably those techniques capable of transforming astarting material containing almost an ilmenite monophase into a productcomposed of two phases (pseudobrookite+rutile) in a stable crystalphase. The oxidation of the starting ore is carried out under conditionssubstantially free of any reductive reaction occurrence.

In carrying out the oxidative treatment, the starting ore is suitablyused in a ground or pulverized form. The ground or pulverized startingmaterials are preferred because such materials can be readily broughtinto contact with an oxidizing agent (e.g. oxygen). The ground orpulverized starting materials may include those prepared according tothe steps of grinding the starting ore to produce a granular or powderyproduct, which is then sieved through a screen to separate a fractionwith a grain or particle size within a certain specified range. Thisrelevant procedure can be carried out with an apparatus generally knownto those skilled in the art. The grain/particle size is not particularlylimited to as long as the oxidative treatment is substantiallyperformable and desired results are obtainable, but it is possible forthose skilled in the art to suitably select an appropriate size. In thisoxidation step, the starting material may be oxidized in a fluidizedstate. It is also preferable to use a reaction apparatus equipped with afluidized bed for bringing the ore particles into contact with anoxidizing agent. The starting ore may be appropriately preheated.

During the oxidative treatments, the starting ore is preferably retainedin the reaction apparatus for a period sufficient to cause the intendedalteration or transformation, for example, for 10 minutes to 200 hours,in sometimes for 20 minutes to 36 hours, and for 40 minutes to 10 hoursin other cases. It is still possible to select an appropriate period oftime according to the specific ore species. The oxidizing agent mayinclude oxygen- or ozone-containing gases, and preferably anoxygen-containing gas. The oxygen-containing gas may include air,mixtures of oxygen with an inert gas, oxygen-enriched air, and others.Typically, this oxidative treatment causes the composition to shift, inthe phase diagram shown in FIG. 5, from the lower right corner obliquelytoward the upper left at an angle of about 45 degrees, finally giving aproduct composed of two coexisting phases (pseudobrookite+rutile).

The resultant product composed of two coexisting phases (pseudobrookite+rutile) is finely pulverized, depending on necessity, to form firstfine particles substantially consisting of pseudobrookite and secondfine particles substantially consisting of rutile. It is not alwaysnecessary that respective fine particles should be absolutelyconstituted of a single crystal phase, but it is only required thatsecond particles substantially consisting of the rutile crystal phasealone or particles containing the rutile crystal phase at a level of atleast 90% or higher can be separated from first particles substantiallyrich in pseudobrookite in a separation step using a magnetic field(magnetic separation step). It is preferable to select a suitableparticle size (grain size) according to the crystal size of the rutilephase formed through the oxidation step. If necessary, the finelypulverized product can be screened or classified through a screen(s) togive a fraction consisting of products within a certain particle sizerange. The pulverization step can be carried out with agrinder/pulverizer known in the art; for example, jet mills, micronizersor the like can be used.

The finely pulverized product is then subjected to the magneticseparation (magnetic fractionation) step using a magnetic field. For themagnetic separation, any magnetic separator known in the art can beused; for example, a magnetic separator suited for the wet or dry methodcan be used. The magnetic separator may include high gradient magneticseparators, magnetic concentrators, drum type magnetic separators andcoupled pole type magnetic separators, and others. It is, however,possible to use a most suitable magnetic separator selected.

The separation utilizing a magnetic field can be done once or repeatedplural times. In some instances, repeated separations are preferred. Themagnetic separation can be carried out in a magnetic field ranging from0.4 to 10 teslas, in some instances from 8×10⁻² to 6 teslas, and from 1to 5 teslas in other cases. Generally, the magnetic field intensityrequired for the magnetic separation is desirably as low as possible butthat intensity can be selected so as to give an optimal separationefficiency depending on crystal sizes and other factors. In cases wherethe magnetic separator is equipped with a filter or the like, optimalmesh sizes of the filter are preferably selected so as to improve themagnetic separation efficiency. It is also preferable to selectadequately any filter mesh size depending on the particle size of thestarting pulverized product. In the magnetic separation, wet methodsare, in some instances, preferred which comprise the steps of suspendingthe starting pulverized product in a fluid medium such as water andsubjecting the suspension to magnetic separation.

The upgraded rutile products (synthetic rutile products) thus-obtainedcan be used in the processes known in the relevant art and in variousconventional titanium product manufacturing processes in which rutileores are used as starting materials. Thus, it is useful for theproduction of titanium by the Kroll process or for pigment producingindustries.

Such combinations with the technologies known in the art also fallwithin the scope of the technology of the present invention.

Details of the present invention are described by the following examplesbut such examples are provided only for illustrative purposes, and forreferential embodiments of the present invention. These examples havebeen described herein for the purpose of illustrating specificembodiments of the present invention but should in no way be construedas limiting and restricting the scope of the invention disclosed herein.It should be understood in the present invention that variousembodiments can be made or executed within the spirit, scope and conceptdisclosed herein

All the examples were carried out or can be carried out, unlessotherwise disclosed herein specifically, by standard techniques whichare well known and conventional to those skilled in the art.

EXAMPLE 1

(1) Phase Equilibrium Determinations at 1373K

In advance, FeTiO₃ was prepared as follows:

Electrolytic iron powders (purity: 99.9 mass %), commercially availableFe₂O₃ and TiO₂ (both: special reagent grade) were weighed (total: 2.5 g,iron: Fe₂O₃: TiO₂=1:1:3 in a molar ratio), and mixed well in an agatemortar. The resultant mixture was compression-molded at about 400 MPa(about 110 kgf/cm²) to give a briquette with a diameter of 10 mm. Thebriquette was in vacuo encapsulated in a quartz tube with an insidediameter of 13 mm to give a capsule, which was heated at 1373K for 3days and then quenched in water to form synthetic FeTiO₃. The productwas ascertained to be FeTiO₃ based on X ray diffraction patterns.

Each test sample was prepared in the following manner:

FeTiO₃ (prepared according to the aforementioned process), commerciallyavailable Fe₂O₃ and TiO₂ (both: special reagent grade) were weighed(total: 2.5 g) at predetermined composition ratios, and mixed well in anagate mortar. The resultant mixture was compression-molded at about 400MPa (about 110 kgf/cm²) to give a briquette with a diameter of 10 mm.The briquette was in vacuo encapsulated in a quartz tube with an insidediameter of 13 mm to give a capsule, which was heated at 1373K for apredetermined period of time for sufficient equilibration and thenquenched in water. After quenching, the sample was taken out of thecapsule and analyzed. The test sample formulations are shown in Table 1and FIG. 1. In FIG. 1, each circle indicates a sample formulation, andeach numeral in the drawing is a sample number.

For the purpose of heating, a vertical type KANTHAL coil electricresistance furnace was used, an X ray diffractometer (XRD) was used foridentifying the phases in each sample and an electron probe X raymicroanalyzer (EPMA) was used for phase composition analysis. EPMAmeasurement results are shown in Tables 2 and 3. TABLE 1 SampleFormulations Sample No. Fe₂O₃ (g) FeTiO₃ (g) TiO₂ (g) Total (g) Fe₂O₃(mol %) FeTiO₃ (mol %) TiO₂ (mol %) 1 1.11515 0.17694 1.20855 2.5001930.00 5.00 65.00 2 0.74898 0.53360 1.21747 2.50005 20.00 15.00 65.00 30.37723 0.89602 1.22669 2.49994 10.00 25.00 65.00 4 1.77496 0.281050.44391 2.49992 60.01 10.00 29.99 5 1.48797 0.56555 0.44664 2.5001650.00 20.00 30.00 6 1.19738 0.85328 0.44929 2.49995 40.00 30.00 30.007(1day)  0.90335 1.14448 0.45194 2.49977 30.00 40.00 30.00 7(3days)0.90351 1.14454 0.45191 2.49996 30.00 40.00 30.00 7(5days) 0.903471.14455 0.45200 2.50002 30.00 40.00 30.00 7(7days) 0.90338 1.144550.45203 2.49996 30.00 40.00 30.00 8 0.60594 1.43932 0.45468 2.4999420.00 50.00 30.00 9 0.19061 1.05051 1.25880 2.49992 5.00 29.00 66.00 100.10650 1.41664 0.97691 2.50005 3.00 42.00 55.00 11 0.16184 1.690860.64751 2.50021 5.00 55.00 40.00 12 0.06481 1.78650 0.64879 2.50010 2.0058.00 40.00(2) Term Requisite for Equilibrium

The sample No. 7 was examined for the relationship between heatingperiods and α phase (hematite ilmenite solid solution (s.s.):ilmenite)/P phase (pseudobrookite s.s: pseudobrookite) compositions. Theexperimental results are shown in FIG. 2. From FIG. 2, it was found thata composition close to the equilibrium composition was already obtainedafter about 2 days (2 d). Therefore, a period of three days (3 d) wasselected as the heating maintenance time for complete equilibriumattainment in other samples differing in composition as well. In FIG. 2,Ni indicates the mole fraction of each component in theFe₂O₃—FeTiO₃—TiO₂ ternary system, and “s.s.” means a solid solutionstate. TABLE 2 EPMA Analysis Results R Phase P Phase α Phase Sample massmass mass mass mass mass No. % Ti % Fe N′_(TiO) ₂ % Ti % Fe N′_(Fe) ₂_(TiO) ₅ N′_(FeTi) ₂ _(O) ₅ % Ti % Fe N′_(Fe) ₂ _(O) ₃ N′_(FeTiO) ₃ 156.28 1.24 1.00 22.82 43.24 0.8572 0.1428 — — — — 2 58.61 0.76 1.0029.83 36.07 0.5273 0.4727 — — — — 3 58.27 0.58 1.00 35.32 30.56 0.27800.7220 — — — — 4 — — — 21.41 45.20 0.9327 0.0673 6.51 63.40 0.78620.2138 5 — — — 24.01 42.17 0.8031 0.1969 11.32 58.09 0.6297 0.3703 6 — —— 27.69 38.72 0.6360 0.3640 16.21 53.25 0.4781 0.5219

TABLE 3 EPMA Analysis Results R Phase P Phase α Phase Sample No. mass %Ti mass % Fe N′_(TiO) ₂ mass % Ti mass % Fe N′_(Fe) ₂ _(TiO) ₅ N′_(FeT)₂ _(iO) ₅ mass % Ti mass % Fe N′_(Fe) ₂ _(O) ₃ N′_(FeTiO) ₃ 7 — — —30.82 35.77 0.4993 0.5007 19.97 49.59 0.3655 0.6345 8 — — — 33.97 31.150.3207 0.6793 26.60 41.95 0.1499 0.8501 9 58.66 0.47 1.00 38.22 27.360.1413 0.8587 — — — — 10 57.44 0.36 1.00 — — — — 31.40 36.89 0.00380.9962 11 57.81 0.71 1.00 38.36 27.09 0.1317 0.8683 30.56 38.28 0.03590.9641 12 58.55 0.43 1.00 — — — — 32.12 36.62 0 1.00 (−0.0112) (1.0112)(3) Phase Relations for the Fe₂O₃—FeTiO₃—TiO₂ System (phase diagram)

The R phase refers to a pure TiO₂ phase, the P phase to a pseudobrookitesolid solution that occurs in the Fe₂TiO₅—FeTi₂O₅ pseudobinary system,and the α phase to an Fe₂O₃—FeTiO₃ solid solution that occurs in theFe₂O₃—FeTiO₃ pseudobinary system. The XRD revealed that Sample Nos. 1,2, 3 and 9 were composed of coexisting two phases, R and P phases;Sample Nos. 4 to 8 composed of coexisting two phases, P and α phases;Sample Nos. 10 and 12 composed of coexisting two phases, R and α phases;and Sample No. 11 composed of coexisting three phases, R, P and αphases. The samples were analyzed with EPMA. The EPMA analysis resultsare shown in Tables 2 and 3. The EPMA analysis was carried out for ironand titanium. The phase diagram of the Fe₂O₃—FeTiO₃—TiO₂ system at 1373Kis shown in FIG. 3. In FIG. 3, the lines connecting black circles aretie lines representing oxygen isobars (activity). Characteristically, ithas been found that FeTi₂O₅ is absent at the temperature of 1373K, aregion wherein three condensed phases R+P+α coexist is present (stippledfield), and said region is fairly narrow. At temperatures higher than1473K, this ternary condensed phase-coexisting region does not occur butthe P phase occurs in the overall composition ranges of theFe₂TiO₅—FeTi₂O₅ pseudobinary system. Therefore, it is considered thatFeTiO₃ is more stable than FeTi₂O₅ at such a low temperature as 1373K.As a result, the ternary condensed R+P+α phase-coexisting region appearsand the solubility range of pseudobrookite s.s. is restricted.

(4) Equilibrium Oxygen Partial Pressure

Each of equilibrated test samples obtained in the manner given in theabove (1) was used as a measurement electrode in an oxygen concentrationcell with a zirconia solid electrolyte, and the equilibriumelectromotive force of each cell thus constituted was measured.

(a) Measurement electrode:

The zirconia electrolyte was a flat-bottomed tube (outside diameter: 15mm, inside diameter: 11 mm, and height: 100 mm) closed at one end andstabilized with 11 mol % CaO. A Pt wire with a coiled end was placed inthe tube, and each test sample was placed thereon and pressed togethertightly. Further used was an 11 mol % CaO-stabilized zirconia solidelectrolyte round-bottomed tube (outside diameter: 10 mm, insidediameter: 7 mm, and height: 50 mm) closed at one end and provided with abottom hole for the passage of a lead wire. This round-bottomed tubeclosed at one end and having a hole bored at the bottom was insertedinto the sample-containing round-bottomed tube closed at one end, andthen pressed against the same by means of a mullite tube (outsidediameter; 5 mm, and inside diameter: 3 mm).

(b) Standard electrode:

Fe was mixed together with FeO at a volume ratio of 1:1 (Fe:FeO), theresulting mixture was placed in an iron crucible with a Pt wire insertedon the inside bottom thereof and compacted for use. The end of the Ptwire was coiled so that the contact thereof with Fe and FeO might beimproved. The aforementioned measurement electrode (flat-bottomed tubeclosed at one end and having an outside diameter of 15 mm) was placed onthis compressed Fe—FeO mixture to constitute a cell.

The thus-constituted cell was placed in a transparent quartz reactiontube, through which purified argon gas was passed at a flow rate ofabout 1.7×10⁻⁶ m³(STP)s⁻¹. Heating was carried out in a Kanthal coilresistance furnace. Electromotive force measurements were done with aprecision DC potentiometer and a digital multimeter with an inputimpedance of 1000 MΩ. After fully stabilized electromotive force valueswere ascertained, each sample was measured at least 10 times at 0.3 ksintervals, and the mean value of observed data was employed as ameasured value. It was ensured from the following fact:

after arrival at the reversible equilibrium potential of the cell, avoltage of several 100 mV was applied to both electrodes for a veryshort period, with the result that the disconnection of impressedvoltage will restore the original equilibrium value and the applicationof a reverse-directional voltage will give the same recovery process.

However, a long period of time was required for arrival at theequilibrium electromotive force; 24 hours or longer in most cells, andseveral hours even in cases where it was attained for a shorter period.

Equilibrium oxygen partial pressures were determined from electromotiveforce data and temperature data, and the relations between the Ti—Fe—Osystem composition and the oxygen partial pressure were estimated. Therelations between the logarithmic oxygen partial pressures and then_(Ti)/(n_(Fe)+n_(Ti)) ratios are shown in FIG. 4 (composition-pressurephase diagram). Here, n_(i) stands for a molar number. In FIG. 4, Rstands for TiO₂, P for the pseudobrookite solid solution (s.s.), α forthe hematite-ilmenite s.s., W for wustite, and Fe for iron.

(5) Upgrading from Natural Ilmenite ores to TiO₂

The compositions of natural ilmenite ores fall within then_(Ti)/(n_(Fe)+n_(Ti)) ratios of from 0.35 to 0.65. When referred to theFe₂O₃—FeTiO₃—TiO₂ system phase diagram as shown in FIG. 3, thissituation can be expressed as shown in FIG. 5.

In view of FIG. 4, ores with an n_(Ti)/(n_(Fe)+n_(Ti)) ratio of nothigher than 0.5 can be reduced to afford Fe+R, namely the R phase (TiO₂)is obtainable as a separate phase. Further, it is seen that ores with ann_(Ti)/(n_(Fe)+n_(Ti)) ratio of not lower than 0.5 can be not onlyreduced but also conversely oxidized even over all oxygen partialpressure ranges to give an intermediate product wherein the R phaseoccurs as a separate phase, such as α+R and P+R.

Thus, the data show that natural ilmenite ores can be upgraded to TiO₂unexpectedly via an oxidation step.

Natural ilmenite ores (with n_(Ti)/(n_(Fe)+n_(Ti)) ratios of 0.5 to0.65) were subjected to oxidative treatments. The oxidative treatmentwas carried out by heating at 1273 to 1473K in air for a predeterminedperiod of time. Upon oxidation of the ores, only the oxygen content ineach ore composition increased, and the composition shifted from thelower right corner in FIG. 5 obliquely toward the upper left in thedirection of about 45 degrees. Thus; the oxidation of ilmenite oresgives products wherein the two phases R+P coexist.

An ilmenite ore mined in Australia was subjected to oxidativetreatments. The ore composition is shown in Table 4 (expressed in % byweight). The titanium content of said ore was about 31.3%, and the Ti/Feratio was 1.19. As for the properties thereof, it occurred in the formof fine particles with a particle size of about 1 mm or smaller. Thisore was heat-treated for oxidation in a porcelain crucible in anelectric furnace open to the atmosphere while varying the retention timeand maintenance temperature.

As a result, the formation of the rutile phase was verified when the orewas heat-treated in air at 1000° C. for 2 days.

The results of X ray diffraction analysis are shown in FIG. 7 incomparison with the results prior to heat treatment. TABLE 4 TiO₂ TFeFeO Fe₂O₃ MnO SiO₂ 52.21 26.4 26.14 16.71 1.73 1.52 Al₂O₃ P₂O₅ Cr₂O₃V₂O₅ MgO CaO 1.00 0.005 0.02 0.12 0.4 0.14(% by weight)

From FIG. 7, it is seen that the raw ore prior to oxidation by heattreatment is constituted of the major constituent ilmenite (FeTiO₃; αphase) and a minor proportion of rutile (TiO₂; R phase), but thermaloxidation in air at 1000° C. for 2 days leads to the disappearance ofthe ilmenite phase and the transformation of constituent crystal phasesinto pseudobrookite (Fe₂TiO₅; P phase) and rutile. This is confirmed tobe as expected from the phase diagrams shown in FIGS. 3 to 5.

(6) Magnetic Susceptibilities of P Phase and R Phase

The magnetic susceptibilities of the ilmenite ore-derived rutile (Rphase) and pseudobrookite s.s. (P phase) were measured. The results areshown in FIG. 6. As compared with the P phase, the R phase was found tohave a negligibly smaller magnetic susceptibility value. Thus, the Pphase can be removed from the oxidized ilmenite ore via magneticallyattracting the same by application of a strong magnetic field of severalteslas. As a result, the R phase (rutile) can be selectively separatedand recovered.

(a) The ore obtained by thermal oxidation for 2 days in air at 1000° C.in the aforementioned step (5) was pulverized into powder with aparticle size of about 0.1 mm or smaller. The powder was subjected to amagnetic separation procedure (magnetic fractionation procedure) with avertical type superconducting magnetic separator. Said magneticseparator is a bismuth oxide superconductor-equipped magnetic separator(owned by “Iwate Industry Promotion Center”, Japan) capable ofgenerating a magnetic field of 10 teslas at maximum, wherein 40stainless-steel mesh filters (mesh size: not larger than 0.5 mm) aresandwiched in a tubular stainless steel separating chamber (tube insidediameter: 20 cm) inserted in a magnetic field generating coil and themeshes themselves are magnetized so that the pseudobrookite phase ismagnetically attracted and the nonmagnetic rutile is obtained from thelower portion of the stainless steel tube.

The magnetic field intensity was set at 5 teslas, and the finelypulverized heat-treated ores were allowed to fall from the stainlesssteel tube upper part of the aforementioned magnetic separator into theseparating chamber for magnetic separation. The pseudobrookite phase wasmagnetically attracted to the stainless mesh filters and soon causedclogging. After removal of all the filters, the powder samples wereagain charged into the separator from the upper part of the stainlesssteel tube. As a result, approximately an half of the powder in a volumeratio was magnetically attracted to the inside wall of the stainlesssteel tube, and the remaining portion fell to the lower portion. Thefraction magnetically adhering to the inside wall of the stainless steeltube was rich in pseudobrookite phase, and the fraction that had fallenwas very rich in rutile.

(b) Wet magnetic separation was done with the same magnetic separator asused in the above process (a).

In this experiment, an aliquot of the same powder sample as used in theabove process (a) was suspended in distilled water (water: powder sampleratio=about 100:5). The suspension was poured from the top of thestainless steel tube, and the separation was carried out in the samemanner as in the above process (a). The initial suspension sampleoccurred as a liquid having a fairly dark brown color owing to the darkbrown color of the pseudobrookite phase. The liquid collected after onepassage through the magnetic separation chamber had a yellowish palebrown color, and the liquid collected after further two passages throughthe magnetic separation chamber occurred as an ocher-colored liquid withan increased degree of whiteness. Thus, the color became white as thenumber of magnetic separation procedures increased. It has been shownthat, upon magnetic separation procedures, the dark brown pseudobrookitephase is preferentially separated by magnetic attraction, and the whiterutile remains in the liquid relatively abundantly, i.e., the P phasecan be magnetically attracted and removed from the oxidized ilmenite oreand the R phase (rutile) can be selectively separated and recovered.

INDUSTRIAL APPLICABILITY

In accordance with the present invention, the inventive process forupgrading ilmenite ores can give a product constituted of rutile (Rphase) and pseudobrookite s.s. (P phase) via oxidation at a relativelylower temperature, as compared to the titanium slag process, and theresultant product can be subjected to a magnetic separation procedurewherein the R phase will be separated from the P phase. Therefore, theamounts of necessary energy and the cost are expected to be markedlyreduced. The process never produces a problem including the formation oflarge amounts of wastes possibly causing environmental pollution. Thetreatment steps involved are only composed of oxidation and magneticseparation steps, which are very simple, and equipment costs can also bereduced.

While the present invention has been described specifically in detailwith reference to certain embodiments and examples thereof, it would beapparent that it is possible to practice it in other forms. In light ofthe disclosure, it will be understood that various modifications andvariations are within the spirit and scope of the appended claims.

1. A process for upgrading an ilmenite ore to produce high-qualitysynthetic rutile products which comprises the steps of: (a) oxidizingthe ilmenite ore to form a stable crystal state type intermediateproduct consisting of two phases, (pseudobrookite+rutile), in a stablecrystal phase, (b) then subjecting the resultant intermediate product toa magnetic separation procedure to separate a rutile phase product as afraction substantially nonresponsive to a magnetic field, and (c)recovering the rutile phase product from others.
 2. The processaccording to claim 1, wherein the ilmenite ore oxidation is carried outwithout allowing any reductive reaction.
 3. The process according toclaim 1, wherein the starting ore almost consisting of monophaseilmenite grains is oxidized to form a stable crystal state typeintermediate product consisting of two phases, (pseudobrookite+rutile),in a stable crystal phase.
 4. The process according to claim 1, whereinthe starting ore is oxidized under non-reducing conditions to form astable crystal state type intermediate product consisting of two phases,(pseudobrookite+rutile), in a stable crystal phase, the resultantintermediate product is then pulverized so as to give (1) fine particlessubstantially composed of pseudobrookite and (2) fine particlessubstantially composed of rutile, the pulverized fine particles aresubjected to a magnetic separation procedure to separate a rutile phaseproduct as a fraction substantially nonresponsive to a magnetic field,and the high-quality rutile phase product was recovered.
 5. The processaccording to claim 4, wherein the pulverization step is carried out onceor repeated plural times.
 6. The process according to claim 1, whereinproducts from the ilmenite ore oxidation step is subjected, underconditions free from any reduction step, to a magnetic separationprocedure wherein products composed of rutile are separated fromproducts composed of pseudobrookite.
 7. The process according to claim1, wherein the magnetic separation procedure is carried out once orrepeated plural times.
 8. The process according to claim 1, wherein theoxidation is carried out at a temperature ranging from 700 to 1200° C.9. The process according to claim 1, wherein the magnetic separationprocedure is carried out under magnetic conditions ranging from 0.4 to10 teslas.
 10. The process according to claim 1, wherein that theresultant rutile phase product has a TiO₂ level of not lower than 90%.11. The process according to claim 1, wherein the process until theproduction of high-quality synthetic rutile products from the ilmeniteore is substantially free of any reduction.